In addition to the separation of responsibilities into different tiers, J2EE also encourages the use of the Model-View-Controller (MVC) design model, briefly introduced in Chapter 3, when designing applications.
The MVC model was first described by Xerox in a number of papers published in the late 1980s in conjunction with the Smalltalk language. This model has since been used for GUI applications developed in all popular programming languages. Let’s review: the basic idea is to separate the application data and business logic, the presentation of the data, and the interaction with the data into distinct entities labeled the Model, the View, and the Controller, respectively.
The Model represents pure business data and the rules for how to use this data; it knows nothing about how the data is displayed or the user interface for modifying the data. The View, on the other hand, knows all about the user interface details. It also knows about the public Model interface for reading its data, so that it can render it correctly, and it knows about the Controller interface, so it can ask the Controller to modify the Model.
Using an employee registry application as an example, an
Employee class may represent a Model. It holds information about an employee: name, employment date, vacation days, salary, etc. It also hold rules for how this information can be changed; for instance, the number of vacation days may be limited based on the employment time. The user interface ...